My wallpaper changes to default image every time I restart my computer.

Step one is to log into a newly created admin user account and see if the problem persists.

Also where is the wallpaper photo stored?

/Library/Screen Savers/Default Collections/

peter_watt wrote:

Also where is the wallpaper photo stored?

The ones assigned to each desktop are stored in a database, here:

~/Library/Application Support/Dock/desktoppicture.db

Or, I guess I should say that pointers to the individual spaces/desktops are set in that database.

Oooopps. I thought you meant screensaver. The windoze-centric "wallpaper" term threw me, since on a Mac they're desktop pictures. Those are in

/Library/Desktop Pictures/.

That was my point. Maybe the OP has the original stored somewhere not accessible at boot up time.

Oh! Misunderstood and thought you were asking for the location.